Proportional output controllers are used in a multitude of applications, including simple "gain" controllers ("Proportional"), and more complex "gain" with "reset" controllers ("Proportional-Integral" or "P-I"), and also yet more complex "gain" with "reset" and "rate" controllers ("Proportional-Integral-Differential" or "P-I-D"). Proportional output controllers have a variable (analog) output, rather than a one-step ON-OFF (digital) output.
Proportional-integral controllers are commonly used to control the speed of motors, the positioning of proportional valves, the feed rate of fuel consuming devices, and many other applications. One use of variable-speed motor control is to control the pressure of a system so as to keep it at a constant setpoint. In such a system, the operator of the system would adjust the setpoint (in this case, the pressure setting) to the desired amount, then the proportional-integral controller would keep the system pressure (the "process variable") very near to that setpoint by either speeding up or slowing down, as required, the motor that drives the pressure-creating device (such as a pump). The sought-after goals of such a control system are: reliable and safe operation, efficient energy usage, stable control, and "tight" control--keeping the process variable very close to the setpoint under all operating conditions. Another goal of every control system is low cost of implementation.
A constant-pressure control system is often difficult to implement properly. Factors such as the location of the pressure transducer and the characteristics of the system load are very important. One very difficult system to control is where the pump (and motor) are turning very slowly, if at all, against a "dead head," where the system load is at the desired pressure (at setpoint) and there is no flow, then a valve is suddenly opened--demanding full flow--with an immediate pressure drop due to the slow or zero speed of the pump. In this circumstance, not only does the mechanical equipment need to be chosen wisely so that it can respond quickly, but the control system must also be able to respond quickly. The variable-speed motor control system must be able to control, with stability, the speed of the motor from full speed all the way down to zero speed in order to perform this application.
There are various P-I and P-I-D controllers available in the prior art. Some of them can perform most of the functions described above, however, many such controllers are quite complex (some use digital techniques, such as microprocessors) and are thus relatively expensive. Other controllers in the prior art use analog techniques, but are still relatively complex (and expensive) in order to perform the wide speed range of control needed to perform the above application.
Analog controllers with proportional outputs in the prior art typically use a variable "threshold" against a constant "ramp" to achieve "phase-control" of an AC sine wave, thereby controlling the amount of energy passed through to the controlled device (a motor, or a valve actuator). The constant "ramp" is timed to start at a low value (near zero volts, for example), then ramp up at a given rate during the half-cycle of each AC sine wave. The "threshold" voltage is determined by the position of the setpoint, and is therefore "variable." Typically, the ramp increases until it reaches the threshold at which time the phase-control "fires," thus turning on an electronic switch which then passes current to the controlled device. If, for any reason, the setpoint control (typically a potentiometer) has a wire break on it, it is very possible for the threshold voltage to then drop to zero volts; if that occurs, the ramp voltage would then exceed the threshold voltage at all times, and the electronic switch would always be "ON." In this situation, the motor (and its driven pump) would then continually run at maximum speed, regardless of its present requirements, leading to a possible safety hazard.
Typical phase-control circuits in the prior art are frequency dependent, i.e., they operate only on one given line frequency, or within a very narrow range of line frequencies. Such circuits are dependent on the "ramp" voltage, which operates at a given time constant. The ramp must fit within each half-cycle of the AC sine wave so the ramp is, therefore, dependent on the line frequency. By the same token, since the ramp function is dependent on the line frequency being a relative constant, then so is the entire phase-control firing circuit dependent on a constant line frequency. In other words, a circuit set up for 60 Hz would probably not run properly on a 50 Hz power source, and definitely not on a 400 Hz power source.